The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to the tuning of such resonators and filters.
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the RF signal is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonant frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonant frequency using this fabrication method. Fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonant frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). The difference between these two types of devices lies mainly in their structures. An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, a FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIG. 1. As shown in FIG. 1, the FBAR device 1 comprises a substrate 2, a bottom electrode 4, a piezoelectric layer 6, a top electrode 8 and a passivation layer 10. The FBAR device 1 may additionally include an acoustic mirror layer 12, which is comprised of a layer 16 of high acoustic impedance sandwiched between two layers 14 and 18 of low acoustic impedance. The mirror usually, but not always, consists of pairs of high and low impedance layers (even number of layers). Some mirror consists of two pairs of such layers arranged in a sequence like SiO2, W, SiO2, W. Instead of the mirror, a FBAR device may additionally include one or more membrane layers of SiO2 and a sacrificial layer. The substrate 2 can be made from silicon (Si), silicon dioxide (SiO2), Galium Arsenide (GaAs), glass, or ceramic materials. The bottom electrode 4 and top electrode 8 can be made from gold (Au), molybdenum (Mo), aluminum (Al), titanium (Ti) or other electrically conductive materials. The piezoelectric layer 6 can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. The passivation layer can be made from SiO2, Si3N4 or polyimide. The low acoustic impedance layers 14 and 18 can be made from Si, SiO2, poly-silicon, Al or a polymer. The high acoustic impedance layer 16 can be made from Au, Mo or tungsten (W), and in some cases, dielectric such as AIN to make a number of layer pairs. FBAR ladder filters are typically designed so that the series resonators yield a series resonance at a frequency that is approximately equal to, or near, the desired, or designed, center frequency of the respective filters. Similarly, the shunt, or parallel, resonators yield a parallel resonance at a frequency slightly offset from the series FBAR resonance. The series resonators are usually designed to have their maximum peak in transmission at the center frequency, so that signals are transmitted through the series resonators. In contrast, the parallel resonators are designed to have their minimum in transmission so that signals are not shorted to ground. FBARs yield parallel resonance and series resonance at frequencies that differ by an amount that is a function of a piezoelectric coefficient of the piezoelectric materials used to fabricate the devices, in addition to other factors such as the types of layers and other materials employed within in the device. In particular, FBAR ladder filters yield passbands having bandwidths that are a function of, for example, the types of materials used to form the piezoelectric layers of the resonators and the thickness of various layers in the device.
The difference in the thickness in various layers in the device can be achieved during the fabrication of the device. Presently, FBARs are fabricated on a glass substrate or a silicon wafer. The various layers in the FBAR-based device are sequentially formed by thin-film deposition. In an FBAR-based device, the resonant frequency of the device usually has to be controlled to within a 0.2-0.5% tolerance. This means that the thickness of each layer in the device must be controlled in the same way. It is known that, however, the deposition of thin-film layers is difficult to control to yield a thickness within such tolerance when the area of substrate or wafer is large. For that reason, manufacturers of FBAR-based devices use wafers of 4-inches or less in diameter for device fabrication. With a small wafer or substrate, certain thickness non-uniformity can be accepted without losing many components due to the operation frequency being out of specification. However, fabricating devices on small wafers or substrates is less cost-effective than doing the same on large substrates. In the case of using large substrates, the problem associated with thickness non-uniformity becomes acute.
Thus, it is advantageous and desirable to provide a method and system to solve the problem associated with thickness non-uniformity in the fabrication of FBAR-based devices on large substrates or wafers.
It is a primary object of the present invention to provide a method and system for achieving the desired resonant frequency of the device within a certain tolerance. This object can be achieved by correcting for the thickness non-uniformity of the devices fabricated on large substrates. The thickness variations can be corrected by selectively removing material from or adding material to the surface area of a wafer (with one or more layers of the device already deposited thereon), or die, before the wafer is cut into individual chips. In that context, the bulk acoustic wave device, as described herein, refers to the entire wafer or substrate that has one or more layers deposited thereon to form one or more individual chips, or part of such wafer or substrate. Moreover, the bulk acoustic wave devices referred to herein include bulk acoustic wave resonators, stacked crystal filters, any combination of the resonators and filters, and the structural variations of the resonators and filters. Furthermore, although one or more layers are already formed on the substrate, the device may or may not have all the necessary layers or the patterns of the layers. For example, the topmost layer on the substrate may be the piezoelectric layer, the top electrode or another layer.
Thus, according to the first aspect of the present invention, a method of tuning a bulk acoustic wave device comprising a substrate and a plurality of acoustic wave generating and controlling layers formed on the substrate, wherein the device has a surface and a thickness, and wherein the device has an operating frequency which varies partly with the thickness of the device and the operating frequency can be adjusted by changing the thickness of the device. The method comprises the steps of:
providing, adjacent to the device, a mask having an aperture;
providing a beam of particles through the aperture for allowing the particles to make contact with the surface of the device in a contacting area substantially defined by the aperture for changing the thickness of the device; and
relocating the aperture of the mask substantially in a lateral direction relative to the device surface for changing the contacting area.
It is preferred that the aperture be larger than one individual chip or a single resonator or filter so that a sufficiently large portion of the device surface is exposed to the beam at once. It is possible, however, to have a small aperture so that only one or two chips or a portion of a chip on the device is exposed to the beam simultaneously.
It is preferable to change the thickness by adding the particles to the contacting area of the device surface. It is also possible to change the thickness by using the particles to remove part of the device surface at the contacting area in a drying etching process.
It is preferable to change the thickness of the device by changing the thickness of the piezoelectric layer. It is also possible to change the thickness of the top electrode or the passivation layer.
Preferably, prior to thickness adjustment, the wafer is mapped to determine the non-uniformity profile of the device surface. Such mapping can be carried out by measuring the frequency of localized areas of the device, or by measuring the thickness of the layer stack. From the mapping result, it is possible to calculate the amount of material to be added to or removed from the device surface as a function of location.
It is understood that if the thickness of the piezoelectric layer is adjusted according to the described method, then a top electrode layer is deposited on the piezoelectric layer after the thickness adjustment. It may be necessary to adjust the thickness of the top electrode layer using the same method. Additionally, a patterning step is usually necessary to produce a desired pattern for the electrode layer. The patterning step is not part of the present invention. Furthermore, if a passivation layer is deposited on top of the patterned top-electrode layer, it may be necessary to adjust the thickness of the passivation layer. Thus, the thickness adjustment steps, according to the present invention, may be carried out one or more times for tuning the entire device, if necessary.
According to the second aspect of the present invention, a system for tuning a bulk acoustic wave device comprising a substrate and a plurality of acoustic wave generating and controlling layers formed on the substrate, wherein the device has a surface and a thickness, and wherein the bulk acoustic wave device has an operating frequency, which varies partly with a thickness of the device, and the operating frequency can be adjusted by changing the thickness of the device. The system comprises:
a mask having an aperture located adjacent to the device;
a source for providing a beam of particles through the aperture for allowing the particles to make contact with the device surface in a contacting area about the aperture for changing the thickness of the device and
means, for relocating the aperture of the mask substantially in a lateral direction relative to the device surface for changing the contacting area.
Preferably, the system also comprises a mechanism of mapping the thickness non-uniformity profile of the device surface prior to adjusting the thickness. Preferably, the mapping mechanism comprises a frequency measurement device for measuring the frequency at different locations of the device surface. It is also possible to use a thickness measurement device to determine the amount of material to be added or removed at different locations.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 2 to 9.